In response to the unexpected shortfall in the 2004-05 influenza vaccine
supply, CDC recommended in October 2004 that vaccine be reserved for persons
in certain priority groups, including persons aged ≥65 years and 6-23 months,
persons aged 2-64 years with conditions that increased their risk for influenza
complications, residents of chronic-care facilities, close contacts of infants
aged <6 months, and health-care workers with direct patient contact.1 In late December 2004, based on declining demand among
these groups, two additional groups (i.e., healthy persons aged 50-64 years
and household contacts of all persons at high risk) were added to the list
of vaccination priority groups.2 To monitor
influenza vaccination coverage during the 2004-05 season, the Behavioral Risk
Factor Surveillance System (BRFSS), an ongoing, state-based, telephone survey
of civilian, noninstitutionalized persons, added new questions to collect
information on priority status and the month and year of vaccination for adults
and children.3 This report is based on analysis
of data collected during February 1-27, 2005, regarding respondent-reported
receipt of influenza vaccination during September 1, 2004–January 31,
2005. The results of this analysis indicated that influenza vaccination coverage
levels through January 2005 among adults in priority groups nearly reached
those in recent years, whereas coverage levels among adults not in priority
groups were approximately half of levels in 2003, in part because 9.3% of
those unvaccinated persons in nonpriority groups declined vaccination this
season. The results further suggested that designation of the priority groups
successfully directed the nation’s influenza vaccine supply to those
at highest risk. In addition, vaccination coverage among children aged 6-23
months was notable (48.4%), given that 2004-05 was the first year this group
was recommended for influenza vaccination.4

In previous years, BRFSS asked adult respondents whether they had been
vaccinated against influenza during the preceding 12 months. No influenza
vaccination questions were asked regarding children, and the only questions
related to high-risk medical conditions referred to diabetes and asthma. To
more closely monitor coverage during this shortfall season, influenza vaccination
questions were added during November 2004–February 2005 regarding children,
priority group status, and month and year of vaccination. For comparison with
the 2004-05 season, data from the 2003 National Health Interview Survey (NHIS)
were used. Similar to the BRFSS survey question, NHIS routinely asks adult
respondents if they received a “flu shot” during the preceding
12 months; NHIS also collects information on occupations and high-risk medical
conditions. NHIS was conducted during 2003 and consisted of in-person interviews;
the household response rate was 89.2%. For children, the only previous available
national data on influenza vaccine coverage were collected in the 2003 National
Immunization Survey (NIS), which reported on vaccination coverage during the
2002-03 season for children aged 6-23 months with an overall response rate
among eligible households of 62.7%.5

Because BRFSS data collection is ongoing, final response rates for February
were not yet available. Preliminary estimates indicate that the median state-level
response rate for February was 51.7% (range: 33.4%-69.8%), based on CASRO
guidelines. Analysis was based on 26,868 interviews from 50 states and the
District of Columbia.

Vaccination Coverage Among Adults

Among adults, influenza vaccination coverage through January of the
2004-05 season was highest among persons aged ≥65 years (62.7%), followed
by health-care workers with patient contact (35.7%) and those aged 18-64 years
with high-risk conditions (25.5%). In comparison, the 2003 NHIS indicated
coverage of 65.6% for persons aged ≥65 years, 40.1% for health-care workers,
and 34.2% for adults aged 18-64 years with high-risk conditions. In contrast,
influenza vaccination coverage among healthy persons aged 18-64 years who
were not health-care workers or contacts of children aged <6 months was
lower than in the previous season (8.8% compared with 17.8%) (CDC, unpublished
data, 2005). Among the reasons cited by respondents for not receiving vaccination,
was “saving vaccine for people who need it more,” cited by 9.3%
of those who were not in priority groups and were not vaccinated. This represents
approximately 17.5 million doses of vaccine potentially made available to
persons in priority groups.

Vaccination uptake was higher in October and November and tapered off
during December and January. Among the adults in the priority groups established
in October, 2% of the vaccinations through January occurred in September,
40% in October, 32% in November, 17% in December, and 9% in January.

Vaccination Coverage Among Children

Influenza vaccination coverage (≥1 doses) among children aged 6-23
months (48.4%) and among children aged 2-17 years with high-risk conditions
(34.8%) was substantially higher than among children not in priority groups
(12.3%). Of the vaccinations received through January, 17% occurred in September,
23% in October, 28% in November, 20% in December, and 12% in January. In comparison,
the 2003 NIS data indicated that coverage among children aged 6-23 months
for the 2002-03 influenza season, before they were recommended for vaccination
by the Advisory Committee on Immunization Practices (ACIP), was 7.4%.5

CDC Editorial Note: During September 1, 2004–January
31, 2005, estimates of influenza vaccination coverage indicate that despite
an unexpected and substantial vaccine shortfall, coverage levels among adults
in the original influenza vaccine priority groups were similar to historical
demand based on the 2003 NHIS,3 thereby suggesting
the effectiveness of prioritization. This resulted, in part, from the estimated
17.5 million persons not in priority groups whose primary reported reason
for not being vaccinated was to save vaccine for people who needed it more.
According to the February 2005 BRFSS, approximately two thirds of the administered
vaccine doses through January went to persons in the initial priority groups
identified in October whereas, during 2003, only approximately one half of
all doses of influenza vaccine were administered to persons in these groups.

The provision of ≥1 doses of influenza vaccination to 48.4% of children
aged 6-23 months during this first influenza season following implementation
of the ACIP recommendations suggests how quickly physicians and parents can
adopt a new disease-prevention guideline.4-6 Because
the Chiron vaccine was not licensed for use in children aged <4 years,
the supply of influenza vaccine for children aged 6-23 months was not affected
by the shortfall.

For the first time, a nationwide, state-based surveillance system (i.e.,
BRFSS) was used to assess influenza vaccination coverage by month of vaccination
and provided the capability to report at intervals as brief as 1 week. This
surveillance system also provided the first national influenza vaccination
coverage estimates for children aged 2-17 years with high-risk conditions.
Having national and state population-based estimates of vaccination coverage
by month and priority status from early in the influenza season afforded policy
makers, health-care providers, public health leaders, and the public timely
information to make decisions regarding distribution and usage of the limited
supply of vaccine.

The findings in this report are subject to at least four limitations.
First, BRFSS is a land-line telephone–based survey and excludes those
segments of the population without telephones or who use only cellular telephones.
Second, data are self-reported and subject to recall bias, particularly for
questions that require recall over a longer period; therefore, for certain
behaviors, prevalence estimates might be under- or overreported. Third, certain
influenza vaccine priority groups were not considered in the survey, including
institutionalized adults and adult caretakers of children aged <6 months
outside of the home (e.g., child care workers). Finally, these results do
not include all of the vaccinations received during the 2004-05 influenza
season. However, based on reports of vaccination, estimated 2004-05 coverage
appeared to increase by less than one percentage point during February among
all the priority and nonpriority groups except those aged 6-23 months, among
whom coverage appeared to increase nearly four percentage points, from 48.4%
to 52.2%.

Comparability of findings from the BRFSS survey with results of the
2003 NHIS is limited because of differences in the survey designs and timeframes.
First, the 2003 NHIS is conducted throughout the entire 2003 calendar year.
Thus, the results reflect vaccinations received anytime during the entire
2002-03 influenza season and vaccinations received during parts of both the
2001-02 and 2003-04 seasons. Second, the interviews are conducted in person,
rather than by telephone. Analysis of 2005 NHIS data, when they become available,
will be helpful to further assess the impact of the 2004-05 vaccine shortfall
and to provide comparisons with results from the February 2005 BRFSS survey.

Vaccination patterns during the 2004-05 influenza season have been affected
by several factors. Although an unexpected and substantial reduction of vaccine
supply occurred at the beginning of the season, prioritization was quickly
recommended and followed. The 2004-05 influenza season was less severe than
the 2003-04 season and did not peak until mid-February.7 In
addition, this was the first full season following the ACIP recommendation
to vaccinate all children aged 6-23 months.

Despite the shortfall of inactivated influenza vaccine, the level of
coverage achieved among those groups prioritized in 2004-05 appears to be
similar to historical coverage. Additional guidelines for prioritization of
influenza vaccination in the event of a future influenza vaccine shortfall
are in development and should assist with efforts to maximize use of available
vaccine.